Scrambling of television video signals is well known. It is often used in cable television systems in connection with the delivery of premium services. Those subscribers who have paid for the premium service are given authorization and key to decoding the scrambled video signal so that the original video image can be displayed on their television screens. More recently, interactive television systems contemplate using cable distribution networks to deliver services on an individualized basis. Such services may include Internet access, electronic mail, catalog shopping or other services which display still frame pictures on a television screen.
A scrambled signal is typically one in which the synchronization signals of the television video signal have been suppressed by baseband or rf sync suppression techniques. Conventional television sets look for the horizontal sync signals of each line to locate the beginning of a line and synchronize the raster display. Horizontal sync signals are provided at the beginning of each line outside the scope of the picture that will be displayed on the television screen. A horizontal sync signal is typically characterized by a negative DC voltage level below that of a voltage level representative of a black image. When displaying a scrambled signal with suppressed horizontal sync signals, the television looking for horizontal sync signals will typically settle upon a sharp transition--from white to black, for example. With a typical moving picture that has been scrambled, the television will lock into a variety of ever changing transitions formed in the actual picture image thus causing the horizontal orientation of each line to vary in space and time. The picture displayed on a television when it is playing a scrambled television video signal that it cannot decode destroys the original image so that it cannot be enjoyably viewed. When the scrambled signal is a still image such as an electronic mail message or a web page, there is a danger that the television will lock onto the beginning of the text or any other still vertical image element such as the edge of a text box or computer window or other framing graphic element. In the case of a textual message, with the transition from a light background to the blacker text remaining fixed in horizontal location over several lines and over time, the television may successfully and repeatedly lock into the same edge during each raster cycle. The danger is that the message may be quite readable even though the sync signals have been suppressed. Thus, the desired level of privacy for the scrambled signals may not be achieved.
Internet access and electronic mail may find prevalence with the advent of interactive cable systems. Aspects of such systems for providing interactive services on a demand basis are described in U.S. Pat. No. 5,550,578; U.S. Pat. No. 5,883,661; U.S. patent application Ser. No. 08/987,405 filed Dec. 9, 1997; and U.S. Provisional Patent Application Ser. No. 60/067,990 filed Dec. 9, 1997, all having the same assignee as the present application. The full disclosure of each of these patent references is hereby incorporated by reference herein.